The present invention relates to modems using QAM modulation techniques and in particular to a circuit for correcting phase jitter and frequency offset impairments.
Phase jitter and frequency offset are two line impairments which have a detrimental effect on the incoming signal a digital modem receives. To prevent these impairments from causing errors in the demodulated modem signal a phase tracking loop may be utilized. Actually, the phase tracking loop is effective with respect to frequency offset and low frequency, low amplitude phase jitter while high amplitude phase jitter sometimes cannot be tracked out by the phase tracking loop.
Heretofore, digital modem designs either contained no phase tracking loop for phase jitter and frequency offset correction or, if they did, they did not decouple the function of the phase tracking loop from the main signal processing function of the modem. As a result, the equalizer error signal would contain the phase shift or jitter components present in the incoming signal.
In view of the above, it is the principal object of the present invention to provide an improved circuit for a digital modem wherein the phase tracking loop function is decoupled from the equalizer function of the modem.
A further object is to provide such a circuit which may be implemented in a conventional digital modem.
Still further objects and advantages will be apparent from the following description of the invention.